Mean free path is the average distance a particle travels between collisions with other particles.

Mean free path occurs in various field ranging from radiography, nuclear physics to even electronics.

However, as far as Thermodynamics is concerned, mean free path refers to the average path length a gas molecule travels between collisions.

Watch this video for a good introduction - https://www.youtube.com/watch?v=0_5cXYaXgVM

Essential idea is to divide the length a gas molecule travels by the expected number of collisions the gas molecule might experience along the way. More details can be found here:

http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/menfre.html